Such an inductive conductivity measurement cell includes, usually, a sending coil, a receiving coil, and a coil support body having an annular, sending coil chamber and an annular, receiving coil chamber, wherein there extends through the coil support body at least a section of a closed media path, which passes through the sending coil chamber and through the receiving coil chamber. As is known to those skilled in the art, an inductive conductivity measurement cell can be described as a combination of two transformers, wherein the closed media path acts as a winding of both transformers. By evaluation of the signal of a receiving coil in the receiving coil chamber in response to the signal of a sending coil in the sending coil chamber, consequently, the conductivity of the medium in the closed media path can be ascertained.
The measuring range of inductive conductivity sensors is, however, limited, especially toward low conductivities, by direct couplings between the sending coil and the receiving coil independently of the measured medium, as will briefly be explained on the basis of an example from the state of the art.
FIGS. 3 and 4 show a perspective view and a sectional view through a coil support body of the state of the art. The coil support body has an annular, sending coil chamber 112 and an annular, receiving coil chamber 114, which are separated from one another by a central, annular partition 119. A corresponding conductivity sensor is available from Endress+Hauser under the designation CLS54.
The grounded coil support body is manufactured of metal and largely shields the coils from one another. However, there is in a mounting area 120 a relatively large bore 122, through which connection lines for the sending coil, on the one hand, and the receiving coil, on the other hand, are led into the sending coil chamber 112 and into the receiving coil chamber 114.
This bore 122 represents a passageway between the sending coil chamber and the receiving coil chamber, whereby a residual coupling between the sending coil and the receiving coil arises, which is independent of the measured medium and, insofar, limits the conductivity measurement range downwards.